…po (#6422)

## Explanation

We identified duplication in JSON RPC middleware logic between the
mobile and extension clients, particularly around the 5792 middleware
stack and capability handling. During recent Wallet API and Perps
integration discussions, the team agreed this presents an opportunity to
abstract and centralize this logic into a core monorepo module.

The motivation here is to:

* Reduce code duplication across mobile and extension clients
* Create a client-agnostic foundation for capabilities (including
auxiliary funds, capability advertising, and required assets)
* Support the upcoming Metamask Pay (MM Pay) initiative by establishing
a standardized middleware layer
* Facilitate future multi-chain and auxiliary funds work without
creating divergent patterns

So the work done on this PR aims to:

1. Create a new core package `eip-5792-middleware` to host shared
middleware logic.
2. Extract the 5792 middleware logic and capability handlers from both
extension and mobile clients.
a.
[Extension](MetaMask/metamask-extension#35541)
   b. [Mobile](MetaMask/metamask-mobile#19064)
4. Refactor them to be client-agnostic and consumable by both clients.
5. Maintain feature parity with existing implementations while improving
modularity and testability.
6. Include initial unit tests and integration hooks.

<!--
Thanks for your contribution! Take a moment to answer these questions so
that reviewers have the information they need to properly understand
your changes:

* What is the current state of things and why does it need to change?
* What is the solution your changes offer and how does it work?
* Are there any changes whose purpose might not obvious to those
unfamiliar with the domain?
* If your primary goal was to update one package but you found you had
to update another one along the way, why did you do so?
* If you had to upgrade a dependency, why did you do so?
-->

## References

<!--
Are there any issues that this pull request is tied to?
Are there other links that reviewers should consult to understand these
changes better?
Are there client or consumer pull requests to adopt any breaking
changes?

For example:

* Fixes
[#5698](MetaMask/MetaMask-planning#5698)
-->

* Fixes
[#5698](MetaMask/MetaMask-planning#5698)

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes